High density integrated circuitry is principally fabricated from semiconductor wafers. An MOS (metal-oxide-semiconductor) structure in semiconductor processing is created by superimposing several layers of conducting, insulating and transistor forming materials. After a series of processing steps, a typical structure might comprise levels of diffusion, polysilicon and metal that are separated by insulating layers. Upon fabrication completion, a wafer contains a plurality of identical discrete die areas which are ultimately cut from the wafer to form individual chips. Die areas or cut dies are tested for operability, with good dies being assembled into separate encapsulating packages which are used in end-products or systems.
CMOS is so-named because it uses two types of transistors, namely an n-type transistor (NMOS) and a p-type transistor (PMOS). These are fabricated in a semiconductor substrate, typically silicon, by using either negatively doped silicon that is rich in electrons or positively doped silicon that is rich in holes. Different dopant ions are utilized for doping the desired substrate regions with the desired concentration of produced holes or electrons.
NMOS remained the dominant MOS technology as long as the integration level devices on a chip was sufficiently low. It is comparatively inexpensive to fabricate, very functionally dense, and faster than PMOS. With the dawning of large scale integration, however, power consumption in NMOS circuits began to exceed tolerable limits. CMOS represented a lower-power technology capable of exploiting large scale integration fabrication techniques.
One type of integrated circuitry comprises memory. The basic unit of semiconductor memory is the memory cell. Capable of storing a single bit of information, the memory cell has steadily shrunk in size to enable more and more cells per area of a semiconductor substrate or wafer. Such enables integrated memory circuitry to be more compact, as well as faster in operation.
Example semiconductor memories include ROMs, RAMs, PROMs, EPROMs and EEPROMs. Some emphasize compactness and economy over speed. Others focus on lightening-fast operation. Some store data indefinitely, while others are so temporary they must be refreshed hundreds of times every second. One of the smallest memory cell comprises the single transistor and single capacitor of a dynamic random access memory (DRAM).